1. Field of the Invention
The invention relates in general to a bias circuit, and more particularly to a reference voltage generation circuit.
2. Description of the Related Art
FIG. 1 shows a reference voltage generation circuit as known in the prior art. As shown in FIG. 1, a conventional reference voltage generation circuit 10 comprises an operational amplifier 101 and resistors 102 and 103. An inverting input end of the operational amplifier 101 is connected between the resistors 102 and 103. The resistor 102 has one end grounded and the other end connected to one end of the resistor 103, which has the other end connected to an output end of the operational amplifier 101. The circuit 10 is applied in a microphone bias circuit (MICBIAS) that demands an extremely low output noise (e.g., smaller than 3 μm), an output voltage between 1.9V and 2.3V, a great current (e.g., greater than 3 mA), and a voltage-temperature change of smaller than 5%. In the prior art, the circuit 10 inputs a low-noise reference voltage Vref at a non-inverting end of the operational amplifier 101 to achieve the above requirements of the MICBIAS. Preferably, a large-size capacitor for inputting the reference voltage Vref at the non-inverting end of the operational amplifier is disposed. At this point, the MICBIAS is in equivalence a buffer. However, due to a large volume of the large-size capacitor, a large space in the MICBIAS is occupied while also increasing costs of the MICBIAS.